Add gitlab-ci automatic docker build to check the pull requests

[elboletaire]

Consider adding this too: https://github.com/sstephenson/bats

[elboletaire]

And / or this too: https://github.com/koalaman/shellcheck

[elboletaire]

I've just changed the title to being gitlab-ci instead of travis. This way we'll be able to test the image with the same build process. So the idea is to define, at least, three stages:

• testing: with bash/shell tests.
• build: builds the image and pushes it to a temporary registry, the same from gitlab will do.
• self-test: use the already built image to check if the sonar scanner still works as expected.